Nerve growth factor sensitivity is broadly distributed among myenteric neurons of the rat colon

Obligatory Activation of SRC and JNK by GDNF for Survival and Axonal Outgrowth of Postnatal Intestinal Neurons

The neurotrophin GDNF acts through its co-receptor RET to direct embryonic development of the intestinal nervous system. Since this continues in the post-natal intestine, co-cultures of rat enteric neurons and intestinal smooth muscle cells were used to examine how receptor activation mediates neuronal survival or axonal extension. GDNF-mediated activation of SRC was essential for neuronal survival and axon outgrowth and activated the major downstream signaling pathways. Selective inhibition of individual pathways had little effect on survival but JNK activation was required for axonal maintenance, extension or regeneration. This was localized to axonal endings and retrograde transport was needed for central JUN activation and subsequent axon extension. Collectively, GDNF signaling supports neuronal survival via SRC activation with multiple downstream events, with JNK signaling mediating structural plasticity. These pathways may limit neuron death and drive subsequent regeneration during challenges in vivo such as intestinal inflammation, where supportive strategies could preserve intestinal function.

Neurotrophic factors in enteric physiology and pathophysiology

Neurotrophic factors are traditionally recognized for their roles in differentiation, growth, and survival of specific neurons in the central and peripheral nervous system. Some neurotrophic factors are essential for the development and migration of the enteric nervous system along the fetal and post-natal gut. Over the last two decades, several non-developmental functions of neurotrophic factors have been characterized. In the adult gastrointestinal tract, neurotrophic factors regulate gut sensation, motility, epithelial barrier function, and protect enteric neurons and glial cells from damaging insults in the microenvironment of the gut. In this issue of Neurogastroenterology and Motility, Fu et al demonstrate that brain-derived neurotrophic factor plays a role in the pathogenesis of distention-induced abdominal pain in bowel obstruction. In light of this interesting finding, this mini-review highlights some of the recent advances in understanding of the physiological and pathophysiological roles of neurotrophic factors in the adult gut.

Vitamins C and E (ascorbate/α-tocopherol) provide synergistic neuroprotection in the jejunum in experimental diabetes

The present study evaluated the synergistic effects of the association of ascorbic acid and α-tocopherol on myenteric in the jejunum of diabetic rats. The rats were randomly divided into four equal groups: untreated normoglycemic (UC), untreated diabetic (UD), ascorbic acid and α-tocopherol-treated normoglycemic (CAE) and ascorbic acid and α-tocopherol-treated diabetic (DAE). The rats from the CAE and DAE group received supplementation with ascorbic acid (1g/L in water) and α-tocopherol (1% in chow). At 210-days-old, the animals were sacrified and their jejunum was collected and submitted to immunohistochemistry. Quantitative and/or morphometric analysis were performed. Supplementation with ascorbic acid and α-tocopherol prevented the cell loss of myenteric neurons expressing HuC/D and TrkA in an equivalent proportion. We also observed a reduction of the CGRP nerve fiber varicosities and the prevention of the increased cell body size of submucosal VIP neurons (p<0.05). The association of ascorbic acid and α-tocopherol reduced the deleterious effects of diabetes promoting protection on the enteric neurons.

Neurotrophin signaling and visceral hypersensitivity

Neurotrophin family are traditionally recognized for their nerve growth promoting function and are recently identified as crucial factors in regulating neuronal activity in the central and peripheral nervous systems. The family members including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) are reported to have distinct roles in the development and maintenance of sensory phenotypes in normal states and in the modulation of sensory activity in disease. This paper highlights receptor tyrosine kinase (Trk) -mediated signal transduction by which neurotrophins regulate neuronal activity in the visceral sensory reflex pathways with emphasis on the distinct roles of NGF and BDNF signaling in physiologic and pathophysiological processes. Viscero-visceral cross-organ sensitization exists widely in human diseases. The role of neurotrophins in mediating neural cross talk and interaction in primary afferent neurons in the dorsal root ganglia (DRG) and neurotrophin signal transduction in the context of cross-organ sensitization are also discussed.

Peripheral contribution of NGF and ASIC1a to colonic hypersensitivity in a rat model of irritable bowel syndrome

BACKGROUND

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder associated with idiopathic colonic hypersensitivity (CHS). However, recent studies suggest that low-grade inflammation could underlie CHS in IBS. The pro-inflammatory mediator nerve growth factor (NGF) plays a key role in the sensitization of peripheral pain pathways and several studies have reported its contribution to visceral pain development. NGF modulates the expression of Acid-Sensing Ion Channels (ASICs), which are proton sensors involved in sensory neurons sensitization. This study examined the peripheral contribution of NGF and ASICs to IBS-like CHS induced by butyrate enemas in the rat colon.

METHODS

Colorectal distension and immunohistochemical staining of sensory neurons were used to evaluate NGF and ASICs contribution to the development of butyrate-induced CHS.

KEY RESULTS

Systemic injection of anti-NGF antibodies or the ASICs inhibitor amiloride prevented the development of butyrate-induced CHS. A significant increase in NGF and ASIC1a protein expression levels was observed in sensory neurons of rats displaying butyrate-induced CHS. This increase was specific of small- and medium-diameter L1 + S1 sensory neurons, where ASIC1a was co-expressed with NGF or trkA in CGRP-immunoreactive somas. ASIC1a was also overexpressed in retrogradely labeled colon sensory neurons. Interestingly, anti-NGF antibody administration prevented ASIC1a overexpression in sensory neurons of butyrate-treated rats.

CONCLUSIONS & INFERENCES

Our data suggest that peripheral NGF and ASIC1a concomitantly contribute to the development of butyrate-induced CHS NGF-ASIC1a interplay may have a pivotal role in the sensitization of colonic sensory neurons and as such, could be considered as a potential new therapeutic target for IBS treatment.

Glial cell line-derived neurotrophic factor is a key neurotrophin in the postnatal enteric nervous system

BACKGROUND

The enteric nervous system (ENS) continues its structural and functional growth after birth, with formation of ganglia and the innervation of growing smooth muscle. However, little is known about factors in the postnatal intestine that influence these processes.

METHODS

We examined the presence and potential role of glial cell line-derived nerve growth factor (GDNF) in the rat postnatal ENS using neonatal tissue, primary co-cultures of the myenteric plexus, smooth muscle, and glial cells as well as cell lines of smooth muscle or glial cells.

KEY RESULTS

Western blot analysis showed that GDNF and its co-receptors rearranged during transfection (RET) and GDNF family receptor alpha-1 were expressed in the muscle layer of the neonatal and adult rat intestine. Immunohistochemistry localized the receptors for GDNF to myenteric neurons, while GDNF was localized to smooth muscle cells. In a co-culture model, GDNF but not nerve growth factor, brain derived neurotrophic factor or neurotrophin-3 significantly increased neuronal survival and more than doubled the numbers of neurites in vitro. RT-PCR, qPCR, Western blotting, ELISA, and immunocytochemistry as well as bioassays of neuronal survival and of RET phosphorylation all identified intestinal smooth muscle as the source of GDNF in vitro. GDNF also induced morphological changes in the structure and organization of neurons and axons, causing marked aggregation of neuronal cell bodies and collinear development of axons. As well, GDNF (50-150 ng mL(-1)) significantly increased [(3)H]-choline uptake and stimulated [(3)H]-acetylcholine release.

CONCLUSIONS & INFERENCES

We conclude that GDNF derived from intestinal smooth muscle cells is a key factor influencing the structural and functional development of postnatal myenteric neurons.

Mitogenic factors promoting intestinal smooth muscle cell proliferation

Intestinal smooth muscle cells are normally quiescent, but in the widely studied model of trinitrobenzene sulfonic acid (TNBS)-induced colitis in the rat, the onset of inflammation causes proliferation that leads to increased cell number and an altered phenotype. The factors that drive this are unclear and were studied in primary cultures of circular smooth muscle cells (CSMC) from the rat colon. While platelet-derived growth factor (PDGF)-AA, fibroblast growth factor (FGF), and epidermal growth factor (EGF) were ineffective, PDGF-BB and insulin-like growth factor-1 (IGF-1) caused significant increase in [(3)H]thymidine incorporation, bromodeoxyuridine uptake, and increased CSMC number, with PDGF-BB (≥0.2 nM) substantially more effective than IGF-1. Surprisingly, CSMC lacked expression of PDGF receptor-β (PDGF-Rβ) upon isolation but by 4 days in vitro, CSMC gained expression of PDGF-Rβ as shown by quantitative PCR, Western blot analysis, and immunocytochemistry; these CSMC responded to PDGF-BB but not IGF-1. PDGF-BB caused PDGF-Rβ phosphorylation and mobilization from the surface membrane, leading to activation of both Akt and ERK signaling pathways, which were essential for subsequent proliferation. In contrast, PDGF-AA, FGF, EGF, and IGF-1 were ineffective. In vivo, control CSMC lacked expression of PDGF-Rβ. However, this changed rapidly with TNBS-colitis, and by day 2 when CSMC proliferation in vivo is maximal, freshly isolated CSMC showed on-going PDGF-Rβ phosphorylation that was further increased by exogenous PDGF-BB. This suggests that the onset of PDGF-Rβ expression is a key factor in CSMC growth in vitro and in vivo, where inflammation may damage intrinsic inhibitory mechanisms and thus lead to hyperplasia.

Nerve growth factor and artemin are paracrine mediators of pancreatic neuropathy in pancreatic adenocarcinoma

OBJECTIVE

To further characterize the neurotrophic attributes of pancreatic cancer (PCa).

SUMMARY BACKGROUND DATA

PCa is characterized by neuropathic alterations which are resulting in pancreatic pain. To further characterize pancreatic neuropathy, we aimed: to analyze whether neuropathic alterations in PCa are only limited to the tumor-core or whether they are similarly encountered in neural structures in the noncancerous pancreas, to demonstrate whether PCa features neurotrophic attributes and finally to identify responsible neurotrophic molecules.

METHODS

Nerve density and area were quantified in normal pancreas (NP, n=45), histologically "normal" pancreas next to pancreatic cancer (NNPCa, n=61) and PCa (n=97). Growth-associated protein-43, nerve growth factor (NGF), and Artemin expressions were assessed by Immunohistochemistry, Western-Blot, and quantitative real time polymerase chain reaction-analyses. Isolated myenteric plexus of newborn rats were exposed to NP, NNPCa, and PCa tissue extracts and supernatants of Panc1 and T3M4 cancer cells with or without Artemin and NGF depletion, followed by neurite density analysis.

RESULTS

Dense neural networks and enlarged nerves were not only detected in PCa but were also present in NNPCa. Growth-associated protein-43, NGF, and Artemin expressions were absent/weak in NP, but increased in both NNPCa and PCa and were closely associated with intrapancreatic neuropathy. PCa and NNPCa tissue extracts and Panc1/T3M4 supernatants noticeably increased neurite density in myenteric plexus-cultures, which were attenuated by depletion of NGF and Artemin.

CONCLUSIONS

The neurotrophic effects of PCa extend into the peritumoral "normal" pancreatic areas without neuro-cancer interactions. The neurotrophic characteristics of PCa can be mimicked by in vitro analyses and reveal NGF and Artemin as potential key players in the generation of pancreatic neuropathy in PCa.

Smooth-muscle-specific expression of neurotrophin-3 in mouse embryonic and neonatal gastrointestinal tract

Vagal gastrointestinal (GI) afferents are essential for the regulation of eating, body weight, and digestion. However, their functional organization and the way that this develops are poorly understood. Neurotrophin-3 (NT-3) is crucial for the survival of vagal sensory neurons and is expressed in the developing GI tract, possibly contributing to their survival and to other aspects of vagal afferent development. The identification of the functions of this peripheral NT-3 thus requires a detailed understanding of the localization and timing of its expression in the developing GI tract. We have studied embryos and neonates expressing the lacZ reporter gene from the NT-3 locus and found that NT-3 is expressed predominantly in the smooth muscle of the outer GI wall of the stomach, intestines, and associated blood vessels and in the stomach lamina propria and esophageal epithelium. NT-3 expression has been detected in the mesenchyme of the GI wall by embryonic day 12.5 (E12.5) and becomes restricted to smooth muscle and lamina propria by E15.5, whereas its expression in blood vessels and esophageal epithelium is first observed at E15.5. Expression in most tissues is maintained at least until postnatal day 4. The lack of colocalization of beta-galactosidase and markers for myenteric ganglion cell types suggests that NT-3 is not expressed in these ganglia. Therefore, NT-3 expression in the GI tract is largely restricted to smooth muscle at ages when vagal axons grow into the GI tract, and when vagal mechanoreceptors form in smooth muscle, consistent with its role in these processes and in vagal sensory neuron survival.

Differential expression of canonical (classical) transient receptor potential channels in guinea pig enteric nervous system

The canonical transient receptor potential (TRPC) family of ion channels is implicated in many neuronal processes including calcium homeostasis, membrane excitability, synaptic transmission, and axon guidance. TRPC channels are postulated to be important in the functional neurobiology of the enteric nervous system (ENS); nevertheless, details for expression in the ENS are lacking. Reverse transcriptase-polymerase chain reaction, Western blotting, and immunohistochemistry were used to study the expression and localization of TRPC channels. We found mRNA transcripts, protein on Western blots, and immunoreactivity (IR) for TRPC1/3/4/6 expressed in the small intestinal ENS of adult guinea pigs. TRPC1/3/4/6-IR was localized to distinct subpopulations of enteric neurons and was differentially distributed between the myenteric and submucosal divisions of the ENS. TRPC1-IR was widely distributed and localized to neurons with cholinergic, calretinin, and nitrergic neuronal immunochemical codes in the myenteric plexus. It was localized to both cholinergic and noncholinergic secretomotor neurons in the submucosal plexus. TRPC3-IR was found only in the submucosal plexus and was expressed exclusively by neuropeptide Y-IR neurons. TRPC4/6-IR was expressed in only a small population of myenteric neurons, but was abundantly expressed in the submucosal plexus. TRPC4/6-IR was coexpressed with both cholinergic and nitrergic neurochemical codes in the myenteric plexus. In the submucosal plexus, TRPC4/6-IR was expressed exclusively in noncholinergic secretomotor neurons. No TRPC1/3/4/6-IR was found in calbindin-IR neurons. TRPC3/4/6-IR was widely expressed along varicose nerve fibers and colocalized with synaptophysin-IR at putative neurotransmitter release sites. Our results suggest important roles for TRPC channels in ENS physiology and neuronal regulation of gut function.

Chagas' disease parasite-derived neurotrophic factor activates cholinergic gene expression in neuronal PC12 cells

A parasite-derived neurotrophic factor (PDNF) produced by the Chagas' disease parasite Trypanosoma cruzi binds nerve growth factor (NGF) receptor TrkA, increasing receptor autophosphorylation, and activating phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK/Erk) pathways, and transcription factor CREB. The end-result is enhanced survival and neuritogenesis of various types of neurons. PDNF also enhances the expression and activity of tyrosine hydroxylase, a rate limiting enzyme in the synthesis of dopamine and other catecholamine neurotransmitters. It remains unknown, however, if PDNF alters expression and metabolism of acetylcholine (ACh), a neurotransmitter thought to play a role in Chagas' disease progression. Here we demonstrate that PDNF stimulates mRNA and protein expression of choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT), which are critical for synthesis and storage of ACh. Stimulation requires functional TrkA because it did not occur in cell mutants that lack the receptor and in TrkA-expressing wild-type cells treated with K252a, an inhibitor of TrkA kinase activity. It also requires TrkA-dependent PI3K and MAPK/Erk signaling pathways because PDNF stimulation of cholinergic transcripts is abolished by specific pharmacological inhibitors. Furthermore, the cholinergic actions of PDNF were reproduced by PDNF-expressing extracellular T. cruzi trypomastigotes at the start of host cell invasion. In contrast, host cells bearing intracellular T. cruzi showed decreased, rather than increased, cholinergic gene expression. These results suggest that T. cruzi invasion of the nervous system alters cholinergic gene expression and that could play a role in neuropathology, and/or lack thereof, in Chagas' disease patients.

Peripheral corticotropin releasing factor (CRF) and a novel CRF1 receptor agonist, stressin1-A activate CRF1 receptor expressing cholinergic and nitrergic myenteric neurons selectively in the colon of conscious rats

Intraperitoneal (i.p.) corticotropin releasing factor (CRF) induced a CRF(1) receptor-dependent stimulation of myenteric neurons and motility in the rat proximal colon. We characterize the colonic enteric nervous system response to CRF in conscious rats. Laser capture microdissection combined with reverse transcriptase polymerase chain reaction (RT-PCR) and immunohistochemistry in longitudinal muscle myenteric plexus whole-mount colonic preparations revealed CRF(1) receptor expression in myenteric neurons. CRF (i.p., 10 microg kg(-1)) induced Fos immunoreactivity (IR) (cells per ganglion) selectively in myenteric plexus of proximal (18.3 +/- 2.4 vs vehicle: 0.0 +/- 0.0) and distal colon (16.8 +/- 1.2 vs vehicle: 0.0 +/- 0.0), but not in that of gastric corpus, antrum, duodenum, jejunum and ileum. The selective CRF(1) agonist, stressin(1)-A (i.p., 10 microg kg(-1)) also induced Fos IR in myenteric but not in submucosal plexus of the proximal and distal colon. Fos IR induced by CRF was located in 55 +/- 1.9% and 53 +/- 5.1% of CRF(1) receptor-IR myenteric neurons and in 44 +/- 2.8% and 40 +/- 3.9% of cholinergic neurons with Dogiel type I morphology, and in 20 +/- 1.6% and 80 +/- 3.3% of nitrergic neurons in proximal and distal colon respectively. CRF and stressin(1)-A elicit defecation and diarrhoea. These data support that one mechanism through which peripherally injected CRF ligands stimulate colonic function involves a direct action on colonic cholinergic and nitrergic myenteric neurons expressing CRF(1) receptor.

Enteric glial cells: new players in gastrointestinal motility?

The enteric glial cells, in addition to being support structures for the enteric nervous system, have many other additional roles, such as modulators for the homeostasis of enteric neurons, cells involved in enteric neurotransmission and antigen-presenting cells. Moreover, in the last years, data have been accumulating that demonstrate a possible active role of these cells in the pathophysiology of gastrointestinal motor activity. Thus, as also shown by recent evidence in both experimental animal models, and in some human diseases, alterations of enteric glial cells might have some role in the development of intestinal motor abnormalities.

Study of expression patterns and levels of neurotrophins and neurotrophin receptors in ulcerative colitis

BACKGROUND

Neurotrophins may be involved in ulcerative colitis (UC). Yet, it is unclear whether if their effects should be blocked.

METHODS

In this study, the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) and their receptors were examined by immunohistochemistry, ELISA, and RT-PCR.

RESULTS

BDNF immunoreaction was detected in nerve structures in particular, and NGF immunoreaction was detected in lamina propria cells. Cellular NGF immunoreaction was generally observed to be higher in the mucosa of UC patients than in the controls. In addition, UC patients demonstrated significantly higher p75 immunoreaction (P = 0.010) in lamina propria cells. The controls expressed significantly higher BDNF immunoreaction in the nerve structures than did UC patients (P = 0.000). However, the UC group showed marked interindividual variation in expression of neurotrophins and neurotrophin receptors. This included variation at the mRNA level for NGF. Differences with the controls were most pronounced in UC specimens demonstrating great infiltration of inflammatory cells and marked tissue derangement. Corticosteroid treatment seemed to affect neurotrophin production in lamina propria cells but not in nerve structures. These observations demonstrate that up-regulation and down-regulation of neurotrophins occur in different structural components in response to the disease process. Massive inflammation seemed to be correlated with decreased neurotrophin immunoreaction in nerve structures, but there was a tendency toward increased neurotrophin production in lamina propria cells.

CONCLUSIONS

Our study shows that UC patients are not a uniform group in their expression of neurotrophins, a fact that should be considered when discussing therapeutic interventions.

Differentiation potential of bone marrow stromal cells to enteric neurons in vitro

OBJECTIVE

To investigate whether bone marrow stromal cells (BMSC) can be induced to differentiate into enteric neurons and to produce more nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF).

METHODS

Bone marrow stromal cells were harvested from male Sprague-Dawley rats and cultured in Dulbecco's modified eagle medium supplemented with 20% fetal bovine serum. The BMSC were passaged six times and characterized by flow cytometry. The BMSC were pre-induced by basic fibroblast growth factor (10 ng/mL) for 24 h, then induced with GDNF in fetal gut condition medium (FGCM) for 10 days. The expressions of neuronal markers neural specific enolase (NSE), neurofilament (NF), glial cell marker, glial fibrillary acedic protein (GFAP), and enteric neuronal marker protein gene product 9.5 (PGP9.5), neural nitric oxide synthase (nNOS), and enteric neural transmitter vasoactive intestinal polypeptide (VIP) were detected by fluorescent immunohistochemistry. Expression levels of GDNF and NGF mRNA were determined by RT-PCR.

RESULTS

The cultured BMSC were CD90 (99.7%) positive and CD45 negative on flow cytometry. At day 10 of induction, 58.5 +/- 10.8% cells adopted neuron-like morphological changes and showed expression of NSE (47.6 +/- 7.5%), NF (75.6 +/- 8.4%), GFAP (negative), PGP9.5 (57.7 +/- 6.5%), nNOS (46.6 +/- 5.4%) and VIP (72.3 +/- 6.7%) by immunofluorescence. The BMSC expressed low levels of NGF and GDNF mRNA; however, after induction of GDNF in FGCM, the expression levels of NGF and GDNF mRNA were significantly increased.

CONCLUSION

Bone marrow stromal cells have the potential to be induced to differentiate into enteric neurons, express enteric neural transmitters, and produce more NGF and GDNF. Therefore, BMSC could be used as new method to treat gastrointestinal motility disorders associated with enteric neural lesions.